Several surface mounted package types are currently in use, including plastic leaded chip carriers (PLCC), plastic quad flat packs, small outline packages and ball grid array (BGA) packages. Each package type supports an IC chip, or "die", and provides interconnections between the die and a printed circuit board (PCB).
FIG. 1 shows an example of a typical molded IC package 10. Lead frame 20 is the central supporting structure of the molded IC package to which all other elements of the molded IC package 10 are attached. Lead frame 20 is etched or stamped from a thin sheet metal strip to form a pattern of narrow leads 22 that radiate from a central die attach platform 24 upon which a die 30 is mounted using, for example, an epoxy resin. Die 30 includes die bond pads 32 which are electrically connected to the radiating leads 22 of the frame by fine-diameter gold wires 40 using well-established wire bond techniques. The assembly including lead frame 20, die 30 and wires 40 are covered with a thermoset plastic casing 45 in an operation called transfer molding. After transfer molding, leads 22 of lead frame 20 are plated, trimmed and formed to complete the molded IC package 10.
As indicated in FIG. 1, a problem with molded IC package 10 is that a distance D between the ends of leads 24 is greater than a length L of plastic casing 45. As such, the printed circuit board (PCB) area taken up by molded IC packages 10 (the package "footprint") includes an apron area which extends around the perimeter of plastic package 45. When several molded IC packages are mounted onto a PCB, the collective apron areas of the mounted IC packages take up a significant amount of PCB space, and prevent a close spacing between the packages. This presents a problem for many applications where it is desirable to minimize the size of the PCB so that it may fit in a small space.
Ball grid array (BGA) packages were developed in part to overcome the spacing problems associated with molded IC packages.
FIG. 2 shows an example of a conventional BGA package 50. A die 60 is attached to the upper surface of substrate 70 using epoxy. Die 60 includes die bond pads 61 which are electrically connected to inner leads 71 of substrate 70 by wires 80 using known wire bond techniques. Substrate 70 includes a plurality of conductive patterns and vias formed in a laminated structure to form electrical connections between inner leads 71 and a set of outer leads (not shown) which are formed on a lower surface of substrate 70. After wires 80 are attached, substrate 70 is overmolded by plastic material 90. BGA package 50 is thereafter mounted onto a host PCB (not shown) using, for example, solder balls 95 which electrically connect outer leads of substrate 70 to electrical connections formed on the host PCB.
Because BGA package 50 is connected to a host PCB using solder balls 95 which are located under substrate 70, the "footprint" of BGA package 50 is substantially the same as the area of substrate 70. This small "footprint" allows close spacing between BGA packages on a PCB, thereby allowing BGA packages to be mounted on smaller PCBs when compared with molded IC packages (discussed above).
However, a problem with BGA packages is that substrates 70 are expensive to produce.
What is needed is a way to formulate a package having the small "footprint" of a BGA package and the low cost of a molded IC package.